XenApi virt driver should throw exception on failure

Virt xenapi driver does not throw InstancePowerOffFailure exception
on a failure while destroying an instance.
_shutdown_instance in compute manager expects InstancePowerOffFailure
and reraises it.
This ensures that instances are not wrongly marked as deleted.

Change-Id: Id57f48a00202bd5ed3125f8e4396a51c12e142cb
Fixes: bug #1190076
Merge "XenApi virt driver should throw exception on failure"